In February 1984, the DTM converted from manual to automated platelet collection techniques, acquiring for this purpose four automated cell separation devices. During the next 9 years, 25 donors undergoing apheresis procedures on the Fenwal CS-3000 device experienced acute hypersensitivity reactions. Fifteen reactions occurred during plateletpheresis and 10 reactions occurred during granulocytapheresis procedures. The purpose of this study was to identify the allergen responsible for these reactions. Using a combination of skin tests, radioallergosorbent tests, and basophil histamine release assays, specific IgE-mediated sensitization to ethylene oxide, a gas used to sterilize the plastic disposable apheresis kits, was found in 10 to 15 plateletpheresis donors and 8 of 10 granulocytapheresis donors experiencing reactions, but in none of 140 nonreacting controls. It is estimated that as many as 1.0% of all repeat apheresis donors may become sensitized to ethylene oxide and experience symptoms during donation. Two of the 10 individuals experienced reactions during granulocyte donations had positive skin tests to hydroxyethyl starch, and appeared to become sensitized to this red cell sedimenting agent during apheresis. Although none of these reactions have been life-threatening, the DTM is continuing to test apheresis donors for sensitization to ethylene oxide and hydroxyethyl starch, and to defer sensitized individuals from subsequent CS-3000 donations.